Non-cake is often gumpaste floral sprays and other toppers, but you should check with the event coordinator, just in case.
Also, start as soon as you can, once you have your design. If you need to do any part of it over, you will have more time.
Unless, it is 'real' cake. Most competitions require using dummies, but there are some that require real cake. for real cake, do as much of the decorations early. Freeze the cake to thaw at the last possilbe moment and still have time to finish.
JoAnnB is right, normally non-cake would mean sugar decorations displayed separately, not on a cake. Sometimes this is called Special Techniques Not on a Cake. Flowers, boxes, sprays, etc., fit into this category.
When designing your cake(s) be sure to keep your board in mind. It may sound odd, but the look of the board goes a long way in a competition. I learned this one the hard way. ALWAYS put a ribbon around the edge of your board.
To make transport and set-up easier, make sure you have some sort of finger lift or cleat under the main board. This is just enough to lift the cake from the table and will allow you to move the cake without damaging it if your hand slips. It also adds a nice lifted look to the cake.
Don't try to decorate for the judges. More than likely, you won't know them and this will not show your ability. Have fun with your entries, decorate them the way you would like them to be. If you do something that you think needs explaining, then explain it. Trust me, the judges will appreciate this.
If you enter a sculpted cake (anything that needs to be carved into a particular shape) then it MUST be real cake and usually three in-progress photos must be included. These photos should show the stacking, carving and decorating process. I have a friend who told me that someone included photos of the batter in the bowl, then in the pan, and then in the oven. This IS in progress, but not exactly what is meant. 
When and where are you entering? I love competitions and would love more information.
Thanks,
Michele
Since it's a 150 year anniversary, then maybe do something about the beginning of the town. You might be able to find something off the wall with a little research. One idea might be to do a bakery type scene or theme for the first bakery of the place.
I don't know all of the rules, but maybe a doll cake with era clothing. How about a farming based cake, an apothecary, a general store, a jail house, or maybe the first court house.
Since I don't know where the place is or the name of the town, I can't really think of anything else. Sorry.
Michele
For a parade, how about a marching band, clowns, and maybe a crowd watching along the street?
How big are you wanting to make the cake and are you planning a stacked cake or a sheet cake? Without knowing the rules, I really don't know how else to help you.
Sorry,
Michele
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